I'm going to do a qualitative usability test of a consumer website in an office.
I assumed it would be better to use a PC computer because I think it is a more popular computer among the general public than Mac. Since I want to try re-create the user experience of consumers, it might be better to use a PC.

PCs are more common. But they may not be more common with your target group of users.
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PhillipWFeb 23 '12 at 10:58

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Why would you want to impose this restriction? The more testing you can do the greater the chance of identifying bugs / issues. If you have the resources and time then certainly test on as many systems, machines, browsers, phones, etc. as you can.
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JonW♦Feb 23 '12 at 11:38

Short answer is that I agree with Viraj; allow the user to use whatever browser/computer they're more comfortable with (if that's feasible), since you want to make the test as authentic as you can. Things like where to find the back button or change tabs or any such thing shouldn't be factors in your test.

Since it's not always feasible to bring two separate computers to a usability test, I often have to make do with my Mac laptop (especially nice because of Silverback) running Windows 7 in Boot Camp. When I do, I find the main thing that helps PC users feel comfortable is ensuring I provide a standard, corded PC mouse. People are not at all comfortable with the trackpad, and especially not the Magic Mouse or even the older Mighty Mouse, often inadvertently right-clicking instead of clicking.

Does the browser make / model / version affect your testing? If it's on a PC, will a certain version of IE behave differently enough to affect your results?

Also, if users are used to using a PC would the chrome of the browser or the physical machine itself be a distraction? If someone had never used an iMac before and sat down at a 27" screen would it distract them in some way from the original test? I would try to recreate the user's environment and toolset as closely as possible.

Agreed. There's a whole raft of variables (which browser; which browser version; whether the site uses other software for video or pdfs; which version of Windows; what bandwidth you provide etc). It's not just a simple 'mac v pc' thing.
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PhillipWFeb 23 '12 at 10:56

PCs are definitely more popular among the general public, but the people visiting the site aren't the general public.

Is the consumer website already active?

If so, you already have data about what percentage of its visitors use a PC vs. a Mac. If you're able to get access to detailed site statistics, you should be able to figure out some of the demographics of its current visitors (age, location, OS, times of day they typically visit, etc.) If you don't have access to the site traffic statistics, some of that information is still publicly available via sites like Alexa.

Your concerns about platform familiarity are worthy but unless the site looks and behaves differently dependant upon platform and browser the problems you are going to identify are unlikely to be related to usability or content findability. Instead they will be routed in the cross platform/browser build quality of the service you plan to test.

If the service requires access and a familiarity with browser functionality then your concerns are again valid. If this is the case the scenario you are concerned with will likely occur as and when users upgrade browsers. For this reason is the real question - To be device and platform agnostic, should the service replicate the browser functionality it requires to allow users to complete tasks from within its own interface?

The differences between Firefox and Chrome on a Mac or Windows machine aren't big (but a few exist here and there). Depending on what versions of IE you need to test on it would be helpful to set up a virtual machine ( check out Virtual Box or whatever you prefer) to install Windows. There is also a package called Multiple IEs that lets you run several versions of IE on a single computer. I found this really helpful for testing older, quirkier versions.

While your answer is correct for cross-browser functional testing (testing that the site behaves to specification across various browsers), the user's question focuses on user testing. Forcing a test participant to use Multiple IEs would be a mistake (and in fact, testing a participant's behaviour on any browser other than the one or two they use themselves would seem to be counterproductive).
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Kit GroseFeb 23 '12 at 2:54

Ahh, you're right. Well Virutal Box still might be a cheap and quick way to provide a Windows and Mac environment for users to test in.
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DorkRawkFeb 23 '12 at 6:14

@KitGrose: The browser should only be a factor when expecting the participant to navigate to the site or do other stuff that requires the browser's functionality. Once within the site or the web-app, the only thing that matters would perhaps be the [Back] button. To avoid "browser intrusion" on the test, Steve Krug recommends running it full screen (F11) which takes the whole browser interface out of the equation.
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Marjan VenemaFeb 23 '12 at 8:14

@MarjanVenema: I don't really agree with that at all. As a user, I open links in new tabs (and new windows) often. I need to be able to reasonably understand where my tabs and windows are. It's my opinion that the test should be as close as possible to the way your real users will experience your product if at all possible.
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Kit GroseFeb 23 '12 at 8:48

@KitGrose: Oh, I do too, and I think most (web) developers / designers do that as well, but the general public apparently doesn't. And according to Jakob Nielsen, a site should not open stuff in other tabs/windows with the exception of "pc documents" such as pdf's (sorry, don't have the link, but it should easily be found on the Nielsen site).
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Marjan VenemaFeb 23 '12 at 10:03

PC - most people will have used a Windows PCs and can adapt easier than people that have not used a Mac. The major issue in the UK is the difference in where the @ sign is. It always manages to 'drop people out of the experience'. So generally I would say PC, but if the software was for a bunch of designers the opposite may be true. But if it is a choice of one or none, then pick any OS, just test it.

I'm working on a large consumer website: the sort of websites most consumers would have used and done online shopping with, at home. I do agree that i should try to re-create the user experience as closely as possible. So I should find out what my sort of computer and browser my users generally use. The site works roughly the same on a mac and PC and on common browsers, but there are subtle differences. also, some users may be intimidated by a Mac if they've never used one before. I'm going to do pilots before the actual tests too.
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SophieFeb 26 '12 at 11:18

A car built to drive smoothly on German roads rarely runs as smoothly on British roads due to subtle differences - mainly that regular frosts break up British roads and do not do the same for German roads. So a good car in Germany may not be so good in Britain.

A PC is a different device to a Mac. As a PC user, it takes a short period of time to adjust to a Mac. The differences albeit subtle do affect how I use the machine and ultimately my end-user experience.